GB 810 730 discloses a small radio transmitter which is worn on the wrist and transmits an audio signal to reveal the wearer's position if he/she is accidentally buried. In todays crowded urban environment however, the transmission of an audio signal alone would result in a high risk of false alarms.
It is also known from GB 1 543 441, U.S. Pat. No. 4,510,350 and U.S. Pat. No. 4,819,860 to provide a wrist mounted transmitter which transmits a digital identification code. In the case of U.S. Pat. No. 4,510,350 the transmitter is triggered to send a prerecorded digitised vocal message over the telephone network by simultaneous depression of two buttons.